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Large Wind Turbines

January 20, 2004
Paul Gipe

Wind Energy Handbook


The Wind Energy Handbook is one of a trio of big wind energy books by John Wiley & Sons'UK office. After a dearth of new wind energy titles in English for nearly a decade, suddenly there are two original books by Wiley, another two English translations of books originally published in German, and a Canadian work on Darrieus turbines.

The Handbook is a massive work that will surely become a classic of its genre. The tome was written by a team of four authors all of whom have first-hand experience with Britain's commercial wind industry. Three of the four worked for the Wind Energy Group, a British manufacturer of wind turbines during the 1980s.

 


Disclosure: My book Wind Energy Comes of Age is published by John Wiley & Sons (US), and the rights to it were briefly held by John Wiley & Sons (UK).


Tony Burton, a civil engineer, was construction manager for WEG when I met him on the site of National Windpower's project at Cemmaes in Wales. Coming from California, I was impressed with WEG's site development practices and rightly or wrongly I've always credited Burton and his client, National Windpower, for a job well done. Today Burton is a consultant to the wind industry from his office in Wales.

Ervin Bossanyi, a controls engineer, wrote several provocative reports on the use of variable speed in the design of wind turbines while at WEG--articles that no doubt made him unpopular with ill-fated U.S. manufacturer Kenetech. Bossanyi also continues to work in the wind field, though now for Garrad Hassan.

David Sharpe is an aerodynamicist who has been a leading figure in British aeronautical circles. He recently retired from his position with the Centre for Renewable Energy Systems Technology at Loughborough University.

Nick Jenkins, also a WEG alumnus, is an electrical engineer who's earlier book, Wind Energy Technology (1997), caught the eye of editors at John Wiley. He's on the faculty at the University of Science and Technology in Manchester.

Wiley, the publisher, took a gamble on wind. At the time in the late 1990s when American publishers were dumping their renewable energy titles, John Wiley & Sons' European division plunged into wind energy. The wind industry had clearly shifted to Europe and Wiley, an American company, followed the market across the Atlantic.

At one time Wiley's UK office had at least three technical books on wind energy in the pipeline. Eventually they published the English translation of Eric Hau's Windkraftanlagen as Large Wind Turbines, Wind Energy Explained, and the Wind Energy Handbook. All three books are big and both Wind Energy Explained and the Wind Energy Handbook are complicated to produce.

There hadn't been a book issued in Britain that comprehensively covered the topic since Leon Freris' Wind Energy Systems by Prentice Hall in 1990, itself a multi-author work by leaders in the British wind community.

Reflecting a trend in publishing, the Handbook is also available electronically. Wiley UK also publishes the Wind Energy Journal, one of only two peer-reviewed journals on wind energy.

"It (the Handbook) covers all the bases," says Andrew Garrad, a principal of international consultants Garrad Hassan. According to Garrad the Handbook "is a worthy successor" to Freris' book, in which Garrad was a contributing author. As the British market expands so to does the need for training, says Garrad, and the Handbook is the kind of resource that will be used to meet the need for advanced training.

The Handbook has high production values. All the charts are sharp and easy to read and there are a number of dramatic photographs of wind turbines in Europe, including pictures of various drive trains as well pitch linkages on variable-pitch turbines.

Reflecting contemporary interest in how wind turbines are being used in the field, the Handbook includes an entire chapter on installation and wind farm design. For American readers, this chapter is useful because it explains the British concept of Zones of Visual Impact as well as viewpoint analysis. It also discusses the sources of wind turbine noise and methods for estimating noise impact.

The Handbook has been long in coming, but it was well worth the wait. Moreover, their timing couldn't have been more on the mark. After nearly a decade in the doldrums, British wind energy is finally stirring. Three companies using continental technology are now manufacturing turbines or components in Great Britain: Vestas in Scotland, NEG-Micon's blade plant in Southampton, and FKI in Loughborough using DeWind's technology. And Britain's first big offshore project is underway. Further evidence that there is--once again--a market for sophisticated engineering texts on wind energy.

Wind Energy Handbook, by Tony Burton, David Sharpe, Nick Jenkins, Ervin Bossanyi, ISBN: 0-471-48997-2, cloth, US $150, 642 pages, 2002. Available from John Wiley & Sons, Baffins Lane, Chichester, West Sussex, PO19 8SQ, United Kingdom, +44 12 43 77 97 77, cs-books@wiley.co.uk, www.wileyeurope.com. An online version of this title is available for license through Wiley InterScience.

 

Acknowledgements
List of symbols
1 Introduction

1.1 Historical Development

1.2 Modem Wind Turbines

1.3 Scope of the Book

References
Bibliography
2 The Wind Resource

2.1 The Nature of the Wind

2.2 Geographical Variation in the Wind Resource

2.3 Long-term Wind-speed Variations

2.4 Annual and Seasonal Variations

2.5 Synoptic and Diurnal Variations

2.6 Turbulence

2.6.1 The nature of turbulence

2.6.2 The boundary layer

2.6.3 Turbulence intensity

2.6.4 Turbulence spectra

2.6.5 Length scales and other parameters

2.6.6 Cross-spectra and coherence functions

2.7 Gust Wind Speeds

2.8 Extreme Wind Speeds

2.8.1 Extreme winds in standards

2.9 Wind-speed Prediction and Forecasting

2.9.1 Statistical methods

2.9.2 Meteorological methods

2.10 Turbulence in Wakes and Wind Farms

2.11 Turbulence in Complex Terrain

References
3 Aerodynamics of Horizontal-axis Wind Turbines

3.1 Introduction

3.2 The Actuator Disc Concept

3.2.1 Momentum theory

3.2.2 Power coefficient

3.2.3 The Betz limit

3.2.4 The thrust coefficient

3.3 Rotor Disc Theory

3.3.1 Wake rotation

3.3.2 Angular momentum theory

3.3.3 Maximum power

3.3.4 Wake structure

3.4 Vortex Cylinder Model of the Actuator Disc

3.4.1 Introduction

3.4.2 Vortex cylinder theory

3.4.3 Relationship between bound circulation and the induced velocity

3.4.4 Root vortex

3.4.5 Torque and power

3.4.6 Axial flow field

3.4.7 Tangential flow field

3.4.8 Radial flow field

3.4.9 Conclusions

3.5 Rotor Blade Theory

3.5.1 Introduction

3.5.2 Blade element theory

3.5.3 The blade element - momentum (BEM) theory

3.5.4 Determination of rotor torque and power

3.6 Breakdown of the Momentum Theory

3.6.1 Free-stream/wake mixing

3.6.2 Modification of rotor thrust caused by flow separation

3.6.3 Empirical determination of thrust coefficient

3.7 Blade Geometry

3.7.1 Introduction

3.7.2 Optimal design for variable-speed operation

3.7.3 A practical blade design

3.7.4 Effects of drag on optimal blade design

3.7.5 Optimal blade design for constant-speed operation

3.8 The Effects of a Discrete Number of Blades

3.8.1 Introduction

3.8.2 Tip losses

3.8.3 Prandtl's approximation for the tip-loss factor

3.8.4 Blade root losses

3.8.5 Effect of tip loss on optimum blade design and power

3.8.6 Incorporation of tip-loss for non-optimal operation

3.9 Calculated Results for an Actual Turbine

3.10 The Aerodynamics of a Wind Turbine in Steady Yaw

3.10.1 Momentum theory for a turbine rotor in steady yaw

3.10.2 Glauert's momentum theory for the yawed rotor

3.10.3 Vortex cylinder model of the yawed actuator disc

3.10.4 Flow expansion

3.10.5 Related theories

3.10.6 Wake rotation for a turbine rotor in steady yaw

3.10.7 The blade element theory for a turbine rotor in steady yaw

3.10.8 The blade element-momentum theory for a rotor in steady yaw

3.10.9 Calculated values of induced velocity

3.10.10 Blade forces for a rotor in steady yaw

3.10.11 Yawing and tilting moments in steady yaw

3.11 The Method of Acceleration Potential

3.11.1 Introduction

3.11.2 The general pressure distribution theory

3.11.3 The axi-symmetric pressure distributions

3.11.4 The anti-symmetric pressure distributions

3.11.5 The Pitt and Peters model

3.11.6 The general acceleration potential method

3.11.7 Comparison of methods

3.12 Stall Delay

3.13 Unsteady Flow - Dynamic Inflow

3.13.1 Introduction

3.13.2 Adaptation of the acceleration potential method to unsteady flow

3.13.3 Unsteady yawing and tilting moments

3.13.4 Quasi-steady aerofoil aerodynamics

3.13.5 Aerodynamic forces caused by aerofoil acceleration

3.13.6 The effect of the wake on aerofoil aerodynamics in unsteady flow References

Bibliography
Appendix: Lift and Drag of Aerofoils

A3.1 Definition of Drag

A3.2 Drag Coefficient

A3.3 The Boundary Layer

A3.4 Boundary-layer Separation

A3.5 Laminar and Turbulent Boundary Layers

A3.6 Definition of Lift and its Relationship to Circulation

A3.7 The Stalled Aerofoil

A3.8 The Lift Coefficient

A3.9 Aerofoil Drag Characteristics

A3.10 Variation of Aerofoil Characteristics with Reynolds Number

A3.11 Cambered Aerofoils

4 Wind-turbine Performance

4.1 The Performance Curves

4.1.1 The Cp - lambda. performance curve

4.1.2 The effect of solidity on performance

4.1.3 The CQ - lambda. curve

4.1.4 The CT - lambda. curve

4.2 Constant Rotational Speed Operation

4.2.1 The Kp -l/lambda curve

4.2.2 Stall regulation

4.2.3 Effect of rotational speed change

4.2.4 Effect of blade pitch angle change

4.2.5 Pitch regulation

4.2.6 Pitching to stall

4.2.7 Pitching to feather

4.3 Comparison of Measured with Theoretical Performance

4.4 Variable-speed Operation

4.5 Estimation of Energy Capture

4.6 Wind-turbine Field Testing

4.6.1 Introduction

4.6.2 Information sources for wind-turbine testing

4.7 Wind-turbine Performance Measurement

4.7.1 Field testing methodology

4.7.2 Wind-speed measurement

4.7.3 Wind-direction measurement

4.7.4 Air temperature and pressure measurement

4.7.5 Power measurement

4.7.6 Wind-turbine status

4.7.7 Data acquisition system

4.7.8 Data acquisition rate

4.8 Analysis of Test Data

4.9 Turbulence Effects

4.10 Aerodynamic Performance Assessment

4.11 Errors and Uncertainty

4.11.1 Evaluation of uncertainty

4.11.2 Sensitivity factors

4.11.3 Estimating uncertainties

4.11.4 Combining uncertainties References

5 Design Loads for Horizontal-axis Wind Turbines

5.1 National and International Standards

5.1.1 Historical development

5.1.2 lEC 61400-1

5.1.3 Germanisher Lloyd rules for certification

5.1.4 Danish Standard OS 472

5.2 Basis for Design Loads

5.2.1 Sources of loading

5.2.2 Ultimate loads

5.2.3 Fatigue loads

5.2.4 Partial safety factors for loads

5.2.5 Functions of the control and safety systems

5.3 Turbulence and Wakes

5.4 Extreme Loads

5.4.1 Non-operational load cases - normal machine state

5.4.2 Non-operational load cases - machine fault state

5.4.3 Operational load cases - normal machine state

5.4.4 Operational load cases - loss of load

5.4.5 Operational load cases - machine fault states

5.4.6 Start-up and shut-down cases

5.4.7 Blade/tower clearance

5.5 Fatigue Loading

5.5.1 Synthesis of fatigue load spectrum

5.6 Stationary Blade Loading

5.6.1 Lift and drag coefficients

5.6.2 Critical configuration for different machine types

5.6.3 Dynamic response

5.7 Blade Loads During Operation

5.7.1 Deterministic and stochastic load components

5.7.2 Deterministic aerodynamic loads

5.7.3 Gravity loads

5.7.4 Deterministic inertia loads

5.7.5 Stochastic aerodynamic loads - analysis in the frequency domain

5.7.6 Stochastic aerodynamic loads - analysis in the time domain

5.7.7 Extreme loads

5.8 Blade Dynamic Response

5.8.1 Modal analysis

5.8.2 Mode shapes and frequencies

5.8.3 Centrifugal stiffening

5.8.4 Aerodynamic and structural damping

5.8.5 Response to deterministic loads-step-by-step dynamic analysis

5.8.6 Response to stochastic loads

5.8.7 Response to simulated loads

5.8.8 Teeter motion

5.8.9 Tower coupling

5.8.10 Wind turbine dynamic analysis codes

5.8.11 Aeroelastic stability

5.9 Blade Fatigue Stresses

5.9.1 Methodology for blade fatigue design

5.9.2 Combination of deterministic and stochastic components

5.9.3 Fatigue predictions in the frequency domain

5.9.4 Wind simulation

5.9.5 Fatigue cycle counting

5.10 Hub and Low-speed Shaft Loading

5.10.1 Introduction

5.10.2 Deterministic aerodynamic loads

5.10.3 Stochastic aerodynamic loads

5.10.4 Gravity loading

5.11 Nacelle Loading

5.11.1 Loadings from rotor

5.11.2 Cladding loads

5.12 Tower Loading

5.12.1 Extreme loads

5.12.2 Dynamic response to extreme loads

5.12.3 Operational loads due to steady wind (deterministic component)

5.12.4 Operational loads due to turbulence (stochastic component)

5.12.5 Dynamic response to operational loads

5.12.6 Fatigue loads and stresses References

Appendix: Dynamic Response of Stationary Blade in Turbulent Wind

A5.1 Introduction

A5.2 Frequency Response Function

A5.2.1 Equation of motion

A5.2.2 Frequency response function

A5.3 Resonant Displacement Response Ignoring Wind Variations along the Blade

A5.3.1 Linearization of wind loading

A5.3.2 First mode displacement response

A5.3.3 Background and resonant response

A5.4 Effect of Ac-Wind Turbulence Distribution on Resonant Displacement Response

A5.4.1 Formula for normalized co-spectrum

A5.5 Resonant Root Bending Moment

A5.6 Root Bending Moment Background response

A5.7 Peak Response

A5.8 Bending Moments at Intermediate Blade Positions

A5.8.1 Background response

A5.8.2 Resonant response References

6 Conceptual Design of Horizontal Axis Wind Turbines

6.1 Introduction

6.2 Rotor Diameter

6.2.1 Cost modelling

6.2.2 Simplified cost model for machine size optimization-an illustration

6.3 Machine Rating

6.3.1 Simplified cost model for optimizing machine rating in relation to diameter

6.3.2 Relationship between optimum rated wind speed and annual mean

6.3.3 Specific power of production machines

6.4 Rotational Speed

6.4.1 Ideal relationship between rotational speed and solidity

6.4.2 Influence of rotational speed on blade weight

6.4.3 Optimum rotational speed

6.4.4 Noise constraint on rotational speed

6.4.5 Visual considerations

6.5 Number of Blades

6.5.1 Overview

6.5.2 Ideal relationship between number of blades, rotational speed and solidity

6.5.3 Some performance and cost comparisons

6.5.4 Effect of number of blades on loads

6.5.5 Noise constraint on rotational speed

6.5.6 Visual appearance

6.5.7 Single-bladed turbines

6.6 Teetering

6.6.1 Load relief benefits

6.6.2 Limitation of large excursions

6.6.3 Pitch-teeter coupling

6.6.4 Teeter stability on stall-regulated machines

6.7 Power Control

6.7.1 Passive stall control

6.7.2 Active pitch control

6.7.3 Passive pitch control

6.7.4 Active stall control

6.7.5 Yaw control

6.8 Braking Systems

6.8.1 Independent braking systems-requirements of standards

6.8.2 Aerodynamic brake options

6.8.3 Mechanical brake options

6.8.4 Parking versus idling

6.9 Fixed-speed, Two-speed or Variable-speed Operation

6.9.1 Two-speed operation

6.9.2 Variable-speed operation

6.9.3 Variable-slip operation

6.9.4 Other approaches to variable~speed operation

6.10 Type of Generator

6.10.1 Historical attempts to use synchronous generators

6.1.0.2 Direct-drive generators

6.11 Drive-train Mounting Arrangement Options

6.11.1 Low-speed shaft mounting

6.11.2 High-speed shaft and generator mounting

6.12 Drive-train Compliance

6.13 Rotor Position with Respect to Tower

6.13.1 Upwind configuration

6.13.2 Downwind configuration

6.14 Tower Stiffness

6.15 Personnel Safety and Access Issues References

7 Component Design

7.1 Blades

7.1.1 Introduction

7.1.2 Aerodynamic design

7.1.3 Practical modifications to optimum design

7.1.4 Form of blade structure

7.1.5 Blade materials and properties

7.1.6 Properties of glass/polyester and glass/epoxy composites

7.1.7 Properties of wood laminates

7.1.8 Governing load cases

7.1.9 Blade resonance

7.1.10 Design against buckling

7.1.11 Blade root fixings

7.2 Pitch Bearings

7.3 Rotor Hub

7.4 Gearbox

7.4.1 Introduction

7.4.2 Variable loads during operation

7.4.3 Drive-train dynamics

7.4.4 Braking loads

7.4.5 Effect of variable loading on fatigue design of gear teeth

7.4.6 Effect of variable loading on fatigue design of bearings and shafts

7.4.7 Gear arrangements

7.4.8 Gearbox noise

7.4.9 Integrated gearboxes

7.4.10 Lubrication and cooling

7.4.11 Gearbox efficiency

7.5 Generator

7.5.1 Induction generators

7.5.2 Variable-speed generators

7.6 Mechanical Brake

7.6.1 Brake duty

7.6.2 Factors govnering brake design

7.6.3 Calculation of brake disc temperature rise

7.6.4 High-speed shaft brake design

7.6.5 Two level braking

7.6.6 Low-speed shaft brake design

7.7 Nacelle Bedplate

7.8 Yaw Drive

7.9 Tower

7.9.1 Introduction

7.9.2 Constraints on first-mode natural frequency

7.9.3 Steel tubular towers

7.9.4 Steel lattice towers

7.10 Foundations

7.10.1 Slab foundations

7.10.2 Multi-pile foundations

7.10.3 Concrete mono-pile foundations

7.10.4 Foundations for steel lattice towers

References
8 The Controller

8.1 Functions of the Wind-turbine Controller

8.1.1 Supervisory control

8.1.2 Closed-loop control

8.1.3 The safety system

8.2 Closed-loop Control: Issues and Objectives

8.2.1 Pitch control

8.2.2 Stall control

8.2.3 Generator torque control

8.2.4 Yaw control

8.2.5 Influence of the controller on loads

8.2.6 Defining controller objectives

8.2.7 PI and PID controllers

8.3 Closed-loop Control: General Techniques

8.3.1 Control of fixed-speed, pitch-regulated turbines

8.3.2 Control of variable-speed pitch-regulated turbines

8.3.3 Pitch control for variable-speed turbines

8.3.4 Switching between torque and pitch control

8.3.5 Control of tower vibration

8.3.6 Control of drive train torsional vibration

8.3.7 Variable-speed stall regulation

8.3.8 Control of variable-slip turbines

8.3.9 Individual pitch control

8.4 Closed-loop Control: Analytical Design Methods

8.4.1 Classical design methods

8.4.2 Gain scheduling for pitch controllers

8.4.3 Adding more terms to the controller

8.4.4 Other extensions to classical controllers

8.4.5 Optimal feedback methods

8.4.6 Other methods

8.5 Pitch Actuators

8.6 Control System Implementation

8.6.1 Discretization

8.6.2 Integrator desaturation References

9 Wind-turbine Installations and Wind Farms

9.1 Project Development

9.1.1 Initial site selection

9.1.2 Project feasibility assessment

9.1.3 The measure-correlate-predict technique

9.1.4 Micrositing

9.1.5 Site investigations

9.1.6 Public consultation

9.1.7 Preparation and submission of the planning application

9.2 Visual and Landscape Assessment

9.2.1 Landscape character assessment

9.2.2 Design and mitigation

9.2.3 Assessment of impact

9.2.4 Shadow flicker

9.2.5 Sociological aspects

9.3 Noise

9.3.1 Terminology and basic concepts

9.3.2 Wind-turbine noise

9.3.3 Measurement, prediction and assessment of wind-farm noise

9.4 Electromagnetic Interference

9.4.1 Modelling and prediction of EMI from wind turbines

9.5 Ecological Assessment

9.5.1 Impact on birds

9.6 Finance

9.6.1 Project appraisal

9.6.2 Project finance

9.6.3 Support mechanisms for wind energy References

10 Electrical Systems

10.1 Power-collection Systems

10.2 Earthing (Grounding) of Wind Farms

10.3 Lightning Protection

10.4 Embedded (Dispersed) Wind Generation

10.4.1 The electric power system

10.4.2 Embedded generation

10.4.3 Electrical distribution networks

10.4.4 The per-unit system

10.4.5 Power flows, slow-voltage variations and network losses

10.4.6 Connection of embedded wind generation

10.4.7 Power system studies

10.5 Power Quality

10.5.1 Voltage flicker

10.5.2 Harmonics

10.5.3 Measurement and assessment of power quality characteristics of grid-connected wind turbines

10.6 Electrical Protection

10.6.1 Wind-farm and generator protection

10.6.2 Islanding and self-excitation of induction generators 10.6.3 Interface protection

10.7 Economic Aspects of Embedded Wind Generation

10.7.1 Losses in distribution networks with embedded wind generation

10.7.2 Reactive power charges and voltage control

10.7.3 Connection charges 'deep' and 'shallow'

10.7.4 Use-of-system charges

10.7.5 Impact on the generation system

References
Index


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